If you think you're doubling down on your sun protection by mixing SPF-infused makeup and sunblock, you might want to check the labels - you could be doing more harm than good.
Mixing different types of sunscreens - namely chemical creams with zinc oxide-based 'mineral' ones - can reduce the efficacy of the sunscreen against UVA rays by up to 91 percent, US researchers have found.
These combinations are often found in makeup products, meaning SPF claims made by foundations or moisturizers may in fact be incorrect.
To conduct the study published this week in Photochemical & Photobiological Sciences, different SPF 15 sunscreen mixtures made of ingredients approved for use in the US and Europe were tested.
The researchers behind the study were particularly concerned by the results of one mixture's UVA protection, the formula of which represented the majority of sun cream mixtures used in the EU and US.
They found when mixed with 6 percent zinc oxide, the chemical sunscreen suffered a 90 percent drop in UVA protection after two hours exposure. The zinc oxide, when exposed to light, degraded the other UV absorbing chemicals in the mixture — reducing its protective qualities.
In comparison, when the formula was exposed to UV for two hours without the zinc oxide it only suffered a 16 percent loss in protective ability.
Professor Richard Blackburn, co-author of the study, said they still recommend consumers use sunscreen "but suggest they should be careful to avoid mixing sunscreen with zinc oxide, whether intentionally with hybrid sunscreens [or] incidentally by mixing sunscreen with other products containing zinc oxide, such as makeup containing SPF."